Home shopping by use of the television has been growing in popularity in recent years. Generally, home shopping channels are transmitted on a community antenna television (CATV) facility. The CATV facility, which has the capacity for transmitting a multiplicity of commercial and public television signals, is usually connected to a large number of homes via coaxial cable. In most of the home shopping systems being offered to date, subscribers passively view the home shopping channel, watch items and pricing being presented by television sales people, and if interested in any particular item, can place an order over the telephone with a sales person. These systems are non-interactive, in the sense that a viewer can only passively watch items as they are presented on the television screen, but cannot control the course of the shopping presentation.
A more advanced interactive home shopping system has been designed and implemented, in which viewers are able to request particular items to be presented for display and can control the shopping presentation as they proceed. A system of this sort is described in U.S. Pat. No. 4,734,764, entitled "Cable Television System Selectively Distributing Pre-recorded Video and Audio Messages". This prior art invention describes a system which conveys still-frame television-quality video with overlaid graphics information and an appropriate audio message (when desired), to a multiplicity of CATV subscribers who tune to a particular cable channel. The subscriber, by use of a Touch Tone telephone, transmits particular codes in response to message prompts which are displayed in menu form on the TV screen, and requests video displays and information on specific products, as well as make purchases. The user of this system requires no additional equipment other than a Touch-Tone telephone and a television.
In order to interactively operate this type of home shopping system, a subscriber tunes to the CATV channel which is being used for transmission, and dials a telephone number to access the system. Each subscriber is given a particular identification number upon subscribing to the service. When this identifying number is entered via the telephone Touch-Tone keypad, the system recognizes the subscriber and his location Based upon succeeding codes which are displayed on the television screen, and which the subscriber enters on the Touch Tone keypad, his television screen begins to display still frame video, having overlaid graphics where appropriate, and possibly accompanied by a sound track to present information which he has requested on an item. Graphic overlays depicting menus and directories of the "electronic stores" which are on the system are also displayed, and by responding to these menus with a sequence of Touch-Tone commands, the subscriber may browse through a particular store of his choice (e.g. a particular aisle in a supermarket), select a particular product of interest, make purchases or request additional information or help in response to prompts on the television screen.
This interactive home shopping system uses a CATV cable network to transmit the video presentations and accompanying audio messages as requested by subscribers. In conventional video transmission, video frames are transmitted at the rate of 30 frames per second (the North American or Japanese standard), or 25 frames per second (the European standard). A video frame is an interleaved composition of two video fields, with each video-field being further composed of a plurality of scan lines referred to as the "vertical blanking interval, and a larger plurality of scan lines which contains the video image information. The interactive home shopping system described in U.S. Pat. No. 4,734,764 makes use of the vertical blanking interval (which consist of the first 21 lines of the video field) to store information which identifies the particular subscriber to whom the requested video and audio data will be sent and his location. The control center of the CATV system (the CATV headend) transmits the video and audio data with this addressing information in the vertical blanking interval down the main "trunk" coaxial cables of the system. In order to compensate for signal losses which naturally occur down the transmission line, most CATV cable systems incorporate amplifiers at strategic locations called "nodes", which are downstream from the control center. At each node an amplifier amplifies the signals from the control center, and transmits the amplified signals down a plurality of secondary distribution cables. Each of the secondary distribution cables is generally provided with a plurality of tertiary distribution cables known as "taps", and finally each of these taps is further split into a plurality of "drop" cables which terminate at the subscriber's home.
In order to accommodate a large number of concurrent subscribers, the interactive shopping system described in U.S. Pat. No. 4,734,764 utilizes a device known as a frame store unit typically located at each node of the distribution system. Each frame store unit services a small number of cable drops, and functions to capture the video frame that is destined for a subscriber whose particular ID code, encoded in the vertical blanking interval, is associated with the unit. Thus, a frame store unit captures video frames having a particular address encoded in the vertical blanking interval of the frame and stores the video information of that frame into its memory. The video frame store then replays the stored video information 30 times per second (according to the U.S. National Television Standards Committee (NTSC) requirement), and transmits the video along with any accompanying audio message to the particular subscriber that it is servicing.
In the prior art system of U.S. Pat. No. 4,734,764, which has been briefly described above, the video and audio data which comprise a particular presentation offering by a merchant, must first be processed and encoded onto conventional laser video discs. A plurality of conventional video disc players at the central system site transmit the appropriate video and audio information in analog form, under control of a central processing unit. This information is then time multiplexed in the proper sequence, and appropriately modulated and frequency converted for transmission down the CATV cable channel.
Numerous problems and limitations are associated with this type of "analog" video display system. First, a large number of video disc players are required, making the cost and physical size of the electronics for the interactive home shopping system exorbitant. Second, the response time between a subscriber keying in a particular code on the telephone keypad and the appearance of a display in response to that code is too slow to provide for a comfortable interactive session. The response time in the analog system is limited by the time it takes for the video disc player to access a particular frame which can be on the order of three seconds. The slow response time is compounded by the graphics overlay process, in which a graphics decoder receives the graphics information that is associated with a particular video frame from the central processing unit, generates the appropriate graphics display data and routes this data to a video combiner, which receives the video frame from the video player and overlays the graphics information onto the video frame.
Further, in the prior art analog system, the audio information is stored on the video disc in the electronic format of the video frame. This imposes a maximum limit of ten seconds for the duration of the audio portion associated with a particular frame. In many cases, this time limitation is too restrictive for practical use.
An additional limitation arises from the use of a laser disc as the storage medium for the video and audio data. A merchant who desires to put a particular presentation for his business onto the interactive home shopping system of the prior art must undertake a lengthy premastering procedure, required to convert his original material (possibly in the format of catalog photographs, video tape information, etc.) into a format which is encoded onto a video disc master. Multiple copies of the master disc must then be made so that each video disc player in the system can have access to the information when it is called upon to deliver a particular frame to a requesting subscriber. This premastering and duplication process is a time-consuming, linear and batch-oriented procedure which provides no mechanism for making minor modifications at a later date Thus, no reusable archiving is possible. If changes are required, a new video disc must be mastered and reproduced.
Finally, the prior art system has general problems which are fundamentally related to storing and copying data in analog form. Analog signals are more prone to degradation by noise sources that arise in any electronic system Further, the maximum signal to noise of the video signals which are attainable at the output of a video disc player is several orders of magnitude below the noise figure for studio quality video broadcast. Degradation of analog signals as they are transmitted down the long lengths of coaxial line which comprises the CATV system is inevitable. This further degrades the video image seen by the subscriber.